Multicast: your bidir RP needs to know that it’s the RP for bidir groups


Today a real no-brainer (especially in hindsight). I had a config where the distribution switch has some vlan interfaces with multicast devices connected to access switches downstream. The problem was that devices on different vlans couldn’t communicate with multicast even though PIM sparse was enabled on both vlan interfaces and RP was set for all groups for bidir traffic. The problem was really simple: if you add the keyword bidir, make sure that on RP you set the same bidir keyword, otherwise it only wants to be an RP for non-bidir multicast traffic.

The lesson here is again very simple: divide the problem into digestible chunks:

  • what vrf are you looking at?
  • is the RP for the same vrf? do you have any RP-address config where you have bidir for some groups only?
  • is sparse mode enabled? or bidir?
  • what can you see in show ip mroute? S,G entries or only *,G entries? why?
  • what can you see in show ip igmp membership tables?
  • where is the RP? what can you see on the RP? Does it know that it is the RP and is PIM enabled on the interface with the IP address that is the RP? Is the same mode enabled (sparse? bidir?) Is it bidir for all groups or only for some?

A bit of research (especially if you deal only occasionally with multicast) doesn’t hurt. That way you know (and you’re not surprised…) why you only see *,G entries when bidir is enabled.



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